Creep-fatigue interaction in a low alloy bainitic steel and an austenitic stainless steel

Creep, fatigue and creep-fatigue tests with five and ten minutes hold time were conducted using smooth and modified keyhole compact tension specimens. Measurements were made of the crack initiation and propagation of a 2 1/4Cr-1Mo steel in the normalized and tempered condition and a 304 stainless steel in the annealed condition at temperature of 565{dollar}spcirc{dollar}C and 650{dollar}spcirc{dollar}C, respectively, in air. Stress redistribution times under creep loading were computed to establish the governing stress field for crack initiation and crack growth using the creep isochronous curves and an analytical solution. Smooth specimen test results were used to predict the crack initiation life under fatigue and creep-fatigue loading using the local strain approach and to predict the total life under creep loading using the reference stress approach. Microstructural analysis was carried out to identify the failure mechanisms.; It was found that crack initiation occupied a major portion of the failure life under creep and creep-fatigue loading and that stress redistribution times were two orders of magnitude lower than the crack initiation times. Both materials were identified as creep-ductile, with reference stress-controlled initiation and growth behavior. Creep failure mechanism predominates after short hold times ({dollar}>{dollar}5 minutes). Creep cavitation was the failure mechanism in 304 stainless steel while exhaustion of matrix ductility with very little creep damage was the dominant failure node for 2 1/4Cr-1Mo steel under creep and creep-fatigue loading.